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INTRODUCTION

More than 100 years ago, the French physiologist Claude Bernard observed that the “milieu interior” (internal environment) of the body remained remarkably constant despite a changing external environment (see A Look Back—Important People in Science). The fact that the body maintains a relatively constant internal environment in spite of stressors (e.g., heat, cold, exercise) is not an accident but is the result of many control systems (15). Control mechanisms that are responsible for maintaining a stable internal environment constitute a major chapter in exercise physiology, and it is helpful to examine their function in light of simple control theory. Therefore, this chapter introduces the concept of “control systems” and discusses how the body maintains a relatively constant internal environment during periods of stress. However, before you begin to read this chapter, take time to review Research Focus 2.1. This box provides an overview of how to interpret graphs and gain useful information from these important tools of science.

Throughout this book, we use line graphs to illustrate important concepts in exercise physiology. Although these same concepts can be explained in words, graphs are useful visual tools that can illustrate complicated relationships in a way that is easy to understand. Let’s briefly review the basic concepts behind the construction of a line graph.

A line graph is used to illustrate relationships between two variables, that is, how one thing is affected by another. You may recall ...